A typical transmission line is of a single preferred characteristic impedance. Any impedance discontinuity anywhere along the line or at either of its terminating ends causes an undesirable reduction or attenuation of signals being transmitted along the line from one end to another. Therefore it is common practice to connect the transmission line between line interface circuits which are designed to provide source and terminating impedances which will compensate for the frequency characteristics of the transmission line.
For example, in the telecommunication industry, a so called E&M trunk includes a pair of wires used for transmitting analog signals, another pair of wires for receiving analog signals, and yet another pair of wires for trunk supervision signals. The usual characteristic impedance of the transmitting and receiving pairs of wires in North America is 600 ohms. However other termination impedances, for example 1200 ohms and 150 ohms are in standard common usage to provide frequency compensation.
The change in terminating impedance to correct for cable frequency response produces a change in the flat loss through the cable. The circuit must therefore change the impedance and provide an accompanying gain change to maintain the same signal level with the improved frequency response.
A line interface circuit may be adaptable to more than one characteristic impedance by providing metallic contact switches for selecting any one of a plurality of terminating resistances. In one example, selection by manual switches may be effected, at any time, to select one of a plurality of operating impedances for the line interface circuit. However, the potential for erroneous selection and the added expense of manual switches are significant disadvantages.
The error potential may be significantly reduced by substituting relay switches for manual switches, whereby the selected impedance of each line interface circuit is effected at a central location, for example, a central control area in a telecommunications facility. However, an added expense is associated with providing appropriate relays and relay driver circuits. A further expense to be born by the operating company is the subsequent and on-going additional floor space and operating power requirement of the relays.